database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-07-30 11:03:19 +03:00
Code Activity

Implement GROUP BY DISTINCT

Browse Source 
With grouping sets, it's possible that some of the grouping sets are
duplicate.  This is especially common with CUBE and ROLLUP clauses. For
example GROUP BY CUBE (a,b), CUBE (b,c) is equivalent to

  GROUP BY GROUPING SETS (
    (a, b, c),
    (a, b, c),
    (a, b, c),
    (a, b),
    (a, b),
    (a, b),
    (a),
    (a),
    (a),
    (c, a),
    (c, a),
    (c, a),
    (c),
    (b, c),
    (b),
    ()
  )

Some of the grouping sets are calculated multiple times, which is mostly
unnecessary.  This commit implements a new GROUP BY DISTINCT feature, as
defined in the SQL standard, which eliminates the duplicate sets.

Author: Vik Fearing
Reviewed-by: Erik Rijkers, Georgios Kokolatos, Tomas Vondra
Discussion: https://postgr.es/m/bf3805a8-d7d1-ae61-fece-761b7ff41ecc@postgresfriends.org
This commit is contained in:
Tomas Vondra
2021-03-18 17:45:38 +01:00
parent cd91de0d17
commit be45be9c33
18 changed files with 333 additions and 27 deletions
Download Patch File Download Diff File Expand all files Collapse all files

54
doc/src/sgml/queries.sgml
View File

@ -1372,6 +1372,55 @@ GROUP BY GROUPING SETS (
</programlisting>
</para>
<para>
<indexterm zone="queries-grouping-sets">
<primary>ALL</primary>
<secondary>GROUP BY ALL</secondary>
</indexterm>
<indexterm zone="queries-grouping-sets">
<primary>DISTINCT</primary>
<secondary>GROUP BY DISTINCT</secondary>
</indexterm>
When specifying multiple grouping items together, the final set of grouping
sets might contain duplicates. For example:
<programlisting>
GROUP BY ROLLUP (a, b), ROLLUP (a, c)
</programlisting>
is equivalent to
<programlisting>
GROUP BY GROUPING SETS (
(a, b, c),
(a, b),
(a, b),
(a, c),
(a),
(a),
(a, c),
(a),
()
)
</programlisting>
If these duplicates are undesirable, they can be removed using the
<literal>DISTINCT</literal> clause directly on the <literal>GROUP BY</literal>.
Therefore:
<programlisting>
GROUP BY <emphasis>DISTINCT</emphasis> ROLLUP (a, b), ROLLUP (a, c)
</programlisting>
is equivalent to
<programlisting>
GROUP BY GROUPING SETS (
(a, b, c),
(a, b),
(a, c),
(a),
()
)
</programlisting>
This is not the same as using <literal>SELECT DISTINCT</literal> because the output
rows may still contain duplicates. If any of the ungrouped columns contains NULL,
it will be indistinguishable from the NULL used when that same column is grouped.
</para>
<note>
<para>
The construct <literal>(a, b)</literal> is normally recognized in expressions as
@ -1560,8 +1609,13 @@ SELECT a "from", b + c AS sum FROM ...
<sect2 id="queries-distinct">
<title><literal>DISTINCT</literal></title>
<indexterm zone="queries-distinct">
<primary>ALL</primary>
<secondary>SELECT ALL</secondary>
</indexterm>
<indexterm zone="queries-distinct">
<primary>DISTINCT</primary>
<secondary>SELECT DISTINCT</secondary>
</indexterm>
<indexterm zone="queries-distinct">

9
doc/src/sgml/ref/select.sgml
View File

@ -37,7 +37,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="parameter">expression</replac
[ * | <replaceable class="parameter">expression</replaceable> [ [ AS ] <replaceable class="parameter">output_name</replaceable> ] [, ...] ]
[ FROM <replaceable class="parameter">from_item</replaceable> [, ...] ]
[ WHERE <replaceable class="parameter">condition</replaceable> ]
[ GROUP BY <replaceable class="parameter">grouping_element</replaceable> [, ...] ]
[ GROUP BY [ ALL | DISTINCT ] <replaceable class="parameter">grouping_element</replaceable> [, ...] ]
[ HAVING <replaceable class="parameter">condition</replaceable> ]
[ WINDOW <replaceable class="parameter">window_name</replaceable> AS ( <replaceable class="parameter">window_definition</replaceable> ) [, ...] ]
[ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] <replaceable class="parameter">select</replaceable> ]
@ -778,7 +778,7 @@ WHERE <replaceable class="parameter">condition</replaceable>
<para>
The optional <literal>GROUP BY</literal> clause has the general form
<synopsis>
GROUP BY <replaceable class="parameter">grouping_element</replaceable> [, ...]
GROUP BY [ ALL | DISTINCT ] <replaceable class="parameter">grouping_element</replaceable> [, ...]
</synopsis>
</para>
@ -802,7 +802,10 @@ GROUP BY <replaceable class="parameter">grouping_element</replaceable> [, ...]
independent <replaceable>grouping sets</replaceable>. The effect of this is
equivalent to constructing a <literal>UNION ALL</literal> between
subqueries with the individual grouping sets as their
<literal>GROUP BY</literal> clauses. For further details on the handling
<literal>GROUP BY</literal> clauses. The optional <literal>DISTINCT</literal>
clause removes duplicate sets before processing; it does <emphasis>not</emphasis>
transform the <literal>UNION ALL</literal> into a <literal>UNION DISTINCT</literal>.
For further details on the handling
of grouping sets see <xref linkend="queries-grouping-sets"/>.
</para>